Batteries and what I've learned

AA are 1.6V, 9v don't currently exist. Capable of high discharge, rechargeable, no memory, will self-discharge very little.

I no longer use Alkaline AA's due to NiZn. They will provide equal or superior performance to Alkaline, while saving you money by the 4th recharge.
The HiPO NiMH 9v's (9.6v, 230-250 MaH) are also fantastic to replace 9V. Just make sure you're getting the good ones, not the cheasy Java (170 MaH although they are the proper 9.6v), or 7.2 or 8.4v that are usually sold in stores.

this is not about battery efficiency but about batteries themselves.. and you can't really say best on batteries cuz someone will come by and say that a pinokio will run 2 9v's through 36 cases.. considering a pinokio uses 9v's and a rotor uses AA's its like comparing apples and oranges when they're in a banana suit..

I know this is an old thread but figure people will still check it out.

If you you use AA or AAA batteries in your hopper I suggest you check out Sanyo Eneploop. they are the only rechargeable batteries that I know of that can sit for 3 years and still maintain 75% of its power. They are considered the best rechargeable at least if they meet your power requirements. I use them in all my remotes and flashlights. They don't yet make a 9V so I use Tenergy in my gun and hopper. Sanyo has recently come out with a new battery for high consumption called Eneloop XX but I have yet to try it.

If anyone is curious, you do not need to have a sophisticated LVC or charge circuit for LiPos. I run them. For all the 9v applications I use 3s(11.1v nominal) with a light resistor. For the 18v packs i just use a 5s. I build my own packs and circuits, I don't buy them. What I use is a simple low voltage indicator light or buzzer. Set it at 3.3 volts and when you hear/see the alarm you still have a while to play and not worry about destroying the battery. If you are really worried about it you can use a kill switch triggered when the LVC sends its alert. Charging is also an absolute breeze. A simple JST plug running to a simple 2/3s LiPo balance charger. Plug it in, when it turns green unplug it. I mount a charger jack that is accessible without having to pull the batteries. Nice thing about the charger I use is that it can be powered by anything from a USB port to a car alternator to a large lantern battery. It means you'll always be able to top it off.

Every LiPo available will have a discharge rate far higher than anything a hopper can use so you don't need to worry about getting weak ones. They are drastically lighter, last significantly longer and greatly improve break out speed. A normal 9v will drop to 6 volts and lower when a load is placed on them. An 11.1 LiPo will not have drop off at all and won't choke the motor.

I'm able to build plug and play LiPo systems for less than $30 including the charger, cells, LVC, balance plug and power leads.